Unidirectional microphone



June 25, 1963 w. A. BEAvERsoN ErAL 3,095,434

UNIDIRECTIONAL MICROPHONE Filed oct. 22. 1959 In Vezzar Warmeeaverson,Eje )Bob erf Cagse! Gfuf; Mer/Wm afzzz ez stef- United States Patent O3,095,484 UNIDIRECIIONAL MICROPHONE Wayne A. Beaverson, Buchanan, andRobert C. Ramsey, Niles, Mich., assignors to Electro-Voice,Incorporated, Buchanan, Mich., a corporation of Indiana Filed Oct. 22,1959, Ser. No. 848,120 Claims. (Cl. 179-1155) The present inventionrelates generally to devices for translating acoustical energy intoelectrical energy, and more particularly to directional microphones.

There are many occasions, particularly in sound recording for motionpictures, sound pickup of television programs, and large stageproductions for radio broadcasts, when it is desirable to keep themicrophone out of the field of action, and it is therefore necessary onsuch occasions to place the mi-crophone at a considerable distance fromthe point at which sound originates. Directional microphones areparticularly advantageous for such uses in order to reduce backgroundnoises and avoid picking up reflected sounds.

Microphones having cardioid response patterns are well known, and thesemicrophones produce negligible response to sounds impinging on themicrophone from the rear of the microphone. The patent application ofAlpha M. Wiggins, entitled Unidirectional Microphone, No. 403,099, tiledJanuary 11, 1954, is an example of a microphone with a cardioid responsepattern.

A microphone with a cardioid response pattern does not have sharpdirectivity, that is, the microphone will respond to sounds occurring oneither side of the microphone as well as in front of the microphone sothat it discriminates principally against those sounds originating atthe rear of the microphone. It is one of the objects of the presentinvention to provide a microphone with greater directivity thanconventional cardioid microphones.

The patents of Harry F. Olson, No. 2,228,886 and No. 2,299,342, discloseanother type of directional microphone which may be referred to as aline microphone. In this type of microphone, a plurality of differentlength acoustical paths extend in front of the microphones so that onlythose sounds traveling directly toward the microphone will arrive at theelectroacoustical transducer of the microphone in proper phase. Modiedforms of line microphones are shown in the patents of Fred Daniels, No.2,739,659 and No. 2,789,651. The line microphones previously known tothe art have been too large and cumbersome for commercial sound, radio,and television use,

the Daniels patents disclosing such a microphone with a length of 1980feet, although it is to be understood that this particular constructionrepresents the longest known line microphone at that time. It istherefore a further object of the present invention to provide adirectional microphone of the line type which is substantially smallerthan those heretofore known to the art for a given directional frequencyrange.

The novel features which are characteristic of the present invention areset forth with particularity in the appended claims. The inventionitself, both as to its organization and method of operation, togetherwith additional objects and advantages thereof, particularly will bestbe understood from the following description of a preferred embodimentthereof, when read in connection with the accompanying drawings, inwhich:

FIGURE l is a side elevational view of a microphone with a preferredconstruction of the present invention;

FIGURE 2 is an enlarged fragmentary sectional view of a portion of themicrophone of FIGURE l taken along the line 2 2 thereof;

FIGURE 3 is an enlarged fragmentary sectional view 3,095,484 PatentedJune 25, 1953 lCC of the microphone of FIGURES 1 and 2 taken on the sameplane as FIGURE 2;

FIGURE 4 is a sectional view taken along the line 4 4 of FIGURE 3;

FIGURE S is a sectional view taken along the line 5 5 of FIGURE 3; and

FIGURE 6 is a sectional view taken along the line 6 6 of FIGURE l.

The following description of the microphone illustrated in the gureswill set forth a pressure gradient electroacoustical transducer with adiaphragm coupled to the atmosphere at its front side by a plurality ofacoustical paths of different lengths, and coupled at its rear sidethrough an acoustical resistance. As a result of this construction, themicrophone has much greater directivity than a line microphone ofcomparable dimensions with a pressure electroacoustical transducer, or acardioid microphone.

The Olson patent, No. 2,228,886, referred to above, includes adisclosure of a line microphone with two forwardly directed acousticaltransmission lines coupled to opposite sides of a pressure gradienttransducer, each of the acoustical transmission lines having a,plurality of acoustical paths of different lengths. The pressuregradient line microphone of the present invention achieves substantiallythe same directivity as the Olson microphone without the complexity andspace requirements of a second multiple path acoustical transmissionline, and also facilitates cancellation of sounds originating behind themicrophone.

FIGURE l illustrates the side view of the microphone and shows amultipath acoustical transmission line 10 in the form of a cylindricaltube extending from the transducer housing 12. The housing 12 isprovided with an outwardly extending circular plate 14 which is adaptedto be journaled within a pair of bifurcations of a mounting stand ofconventional construction.

The acoustical transmission line 10 must provide a plurality ofdifferent length acoustical paths to the transducer within the housing12. In the construction shown in FIGURES 1 and 2, this is achieved byproviding a sound permeable window in the form of a plurality of holes16 equally spaced along a line parallel with the axis of a hollowcylindrical tube 18. One end of the tube 18 is sealed within the housing12 for the transducer, and the other end of the tube 18 is sealed by anend plug 20. The end plug 2) is also provided with a bore 22 extendingtherethrough to permit sound waves to enter into the tube from the endthereof. The bore 22 particularly aids the response and performance ofthe microphone when used for close talking.

The acoustical transmission line 10 also has a second hollow cylindricaltube or sleeve 24 coaxially disposed about the tube 18 and spacedtherefrom. The sleeve 24 is provided with a number of elongated slots 26equally spaced along two lines disposed on opposite sides of the sleeveand displaced from the line of the holes in the tube by degrees relativeto the common axis. The purpose of the sleeve 24 is to keep dirt andforeign particles out of Vthe tube 18, and otherwise protect the tube.For this reason, the slots 26 are provided with sound permeable Windows28, such as a cloth layer.

'The holes 16 in the tube 1S are provided with a layer of material 30,as illustrated in FIGURE 6, which forms with the holes 16 an acousticalimpedance. The acoustical impedance of the holes adjacent to the housing12 iS greater than that of the remote holes 16 and layers 30 so that theholes 16 act as an extended opening parallel to the axis of the tube,and sound pressures exerted at all points along the tube 13 result inequal sound pressures in the housing 12.

The plug 20 has an axial threaded channel 32 which accommodates amounting screw 34 for a sound permeable cap 36. rl`he cap 36 has acircular rim 37 with an indentation 38 which fits about the exteriorsurface of the sleeve 24, and `an axial hub 40 which is `attached to therim 37 by a disc portion 42. The disc portion has an opening 44-extending therethrough and aligned with the bore 22 in the plug 20, anda sound permeable screen 46 extends from the hub 40 `to the rim 37 tokeep particles out of the tube 13.

FIGURES 3, 4 and 5 illustrate the details of the housing 12 and theele-ctrocoustical transducer disposed within the housing, designated 48.The housing 12 has a cyilindrical cross section and `an axial opening 50at one end which engages the end of the sleeve 24 opposite the cap 36.The housing 12 also has a circular shoulder 52 which extends inwardlyfrom the opening 5t) to engage the end of the tube 18 opposite the cap36. The transducer 48 has a cylindrical permanent magnet 54 which has achannel 56 therethrough which has the same diameter as the tube 13 andis aligned with the tube 18. The transducer 48 also has a yoke 58 whichhas a leg 68 secured to the housing 12 and disposed between the magnet54 and the housing 12, the leg 60 being integral with a second leg 62normal thereto extending parallel to the magnet S4. A magneticallyimpermeable hollow sleeve 64 is disposed iabout the magnet 54 andextends from the leg 6i] of thc yoke 58 to a plate 66 of magneticallypermeable material which completes the yoke.

The plate 66 has a cylindrical. opening therein confronting the magnet54 and spaced therefrom to form a circular gap 70. A domed diaphragm 72is mounted at its periphery on the plate 66 and is positioned to sealthe channel 56 ol' the magnet, so that the diaphragm 72 is coupled 0none side to the acoustical transmission line 10. The diaphragm 72supports a voice coil 74 which is disposed within the gap 70 andthevoice coil is translatable parallel to the axis of the magnet responsiveto sound pressures from the acoustical transmission line 10. It is to beunderstood that some other electromechanical transducer may be employedwith the diaphragm 72 rather than the electromagnetic transducerdisclosed, such as a piezoelectric electromechanical transducer.

A plate 76 is mounted to the peripheral portions of the plate 66 of themagnetic yoke, and the plate 76 is provided with an indentation 78confronting the dome of the diaphragm 72. The plate 76 is constructed ofnonmagnetic material, and it is provided with two legs 80 on oppositesides thereof which extend normal thereto. The legs 80 abut oppositesides of a disc 82 which is secured to the housing 12 by screws 83anchored in the housing. and thus the legs 80 transmit pressure tomaintain the transducer 48 in position.

A transformer 84 for matching the impedance of the transducer voice coilto `an electrical transmission line iS mounted to the side of the disc82 opposite the transducer 48, and a hollow cover 86 is disposed aboutthe transformer 84. The cover 86 is cup shaped and sealed to the housingand maintained in position by a bolt 88 extending through an axialopening 89 in the cover 88 and anchored in a threaded bore 90 on theaxis of the disc 82. As illustrated in FIGURE 4, the cover 86 isprovided with a slot shaped opening 92 on its surface adjacent to thecircular plate 14 of the mounting means for the microphone, and a soundpermeable covering 94 is disposed over the slot 92.

Sound pressures are free to enter into the interior of the cover 86through the slot 92. The disc 82 then permits the sound pressures topass through two small bores 96 in the disc, shown in FIGURE 4. Thebores 96 are spaced from each other and located remote from thetransformer. A layer 98 of cloth is disposed over the bores 96. Theplate 76 is also provided with a plurality of holes 99 disposed in anarc which confront the periphery of the diaphragm dome. The holes 99 arealso covered with a cloth layer 100. FIGURE 5 also illustrates two holes102 which accommodate the leads to the voice `coil 74 from thetransformer 84.

The slot 92, interior of the cover 86, bores 96 in the disc 82, andholes 99 in the plate 76 form a path for sound pressures to the side ofthe diaphragm 72 opposite the acoustical transmission line 10. Thisacoustical path is provided with a irst acoustical impedance formed bythe bores 96 and layers 9S, and a second acoustical impedance formed bythe holes 99 and layers 10i). This construction is designed to performthree separate functions.

First, the acoustical path is provided with the proper phase shift andamplitude attenuation to provide a cardioid polar response pattern forthe lower frequencies of the response range of the microphone. The firstaccoustical impedance and the chamber between the plate 76 and disc 82are most prominent in providing the desired phase shift and amplitudeattenuation for this purpose.

Second, the first and second acoustical impedances provide a relativelysharp `frequency cut-oli for the acoustical path at a frequency from 200cycles per second to 800 cycles per second, thus preventing soundpressures of frequencies above the cut-oft" frequency from impinging onthe side of the diaphragm opposite the transmission line 10. Forfrequencies above the cut-off frequency, the microphone operates as aline microphone.

Third, the second acoustical impedance and the chamber between the plate76 and disc 82, and to a lesser extent the first acoustical impedanceand the chamber within the cap 86, damp the mechanical resonance of thediaphragm 72.

ln one particular construction of a microphone according to theteachings of this invention, the tube 18 has an outer diameter of 1/2inch and has a length of 11g/32 inches. Thirty-two holes 16 are spacedby a distance of llg inch along the tube, and the diameter of the holesis approximately 0.094 inch. The sleeve 24 is spaced from the tube `18by approximately 1A inch, and four slots 26 are disposed in each side ofthe sleeve 24 with a width of W16 inch and a length of 21/2 inches. Thetube 18 is constructed of steel and the sleeve 24 is constructed ofbrass, although it is to be understood that other materials are alsosuitable.

Operation of the microphone may be described as follows. The acousticaltransmission line `1t) provides a plurality of paths `to the diaphragm72 of different length, each path being delined by one of the holes 16.For sounds traveling along the axis of the microphone from the frontthereof, the total path length to the diaphragm is the same for allholes 16 through which the sound may enter the transmission line 10.Therefore, each of the acoustical paths delivers sound energy to thediaphragm which is of the same phase, and there is no cancellation ofsound on the front side of the diaphragm. Sound energy directed towardthe microphone from any other direction than the front, passes throughthe plurality of acoustical paths of the transmission line 10 withditierent total length paths, which results in phase differences andcancellation at the front of the diaphragm provided the line is at leasta half wavelength along.

If the transmission line 10 is less than a half wavelength in length,there will be incomplete cancellation at the diaphragm of soundsoriginating oil of the forward axis of the microphone. For this reason,the cut-off frequency of the path to the rear of the microphone isselected to be approximately equal to the frequency at which thetransmission line 10 is a half wavelength. As a result, the microphoneis directional above the cut-off frequency by virtue of the directivityof the acoustical transmission line, and directional below the cut-offfrequency by virtue of the cardioid response resulting from the soundpath to the rear of a diaphragm. Hence, the microphone is directionalthroughout its entire response range, even though the acousticaltransmission line is too short to produce significant directivity at thelower frequencies of the response range.

In the frequency range near the cut-olf frequency, both the cardioidresponse from the sound path to the rear of the diaphragm and thedirectional charcteristics of the acoustical transmission line occur,and these are combined at the diaphragm to maintain the directivity ofthe microphone. ln this frequency range, sounds originating at the rearof the microphone are not completely cancelled by the multi-pathtransmission line l0, and sound pressure passing through the path to therear of the diaphragm is beneficial to provide this cancellation.

From the foregoing disclosure, those skilled in the art will readilydevise many modifications and improvements upon the microphone describedherein which are within the spirit of this invention. For example, themulti-path transmission line can clearly be formed by a single slot inthe tube 18, rather than a plurality of holes 16 spaced along the tube;or the transmission line could beconstructed by a plurality of separatetubes of dilferent length, as described in the art. -It is thereforeintended that the scope of the present invention be not limited by theforegoing disclosure, but rather only by the' appended claims.

The invention claimed is:

l. A microphone responsive through a frequency range comprising anelectroacoustical transducer having a diaphragm, said transducergenerating an electrical signal responsive to the dilerence in pressuresexerted on opposite sides of the diaphragm, a casing enclosing saidtransducer and having a portion defining at least one acousticaltransmission line extending from one side ot the diaphragm, said casinghaving means located at a plurality of distances from the diaphragm toadmit sound into the transmission line, thus forming a plurality ofsound paths to the diaphragm of different lengths, the longest of saidpaths being less than a half wavelength longer than the shortest of saidpaths at the lowest frequency of the response range, said casingincluding means defining an acoustical path extending from the otherside of the diaphragm having a cavity and an opening on said other sideof the diaphragm spaced from the diaphragm by a distance less thanone-half the length of the longest path of the transmission line, saidpath having an acoustical impedance disposed therein and a band passextending from the lowest frequency of the range of the microphone tothe frequency at which the longest of the paths of the transmission lineis a half wavelength, the volume of said cavity and the magnitude ofsaid impedance comprising means for producing an acoustical phase shiftapproximately equal to the shift in phase of a sound wave in said passband traveling between the diaphragm and opening whereby the microphoneachieves a directional response in ythe portion ol" the frequency rangeof the microphone covered by the band pass of said acoustical path tosaid other side of the diaphragm.

2. A microphone comprising the elements of claim l wherein the meansdening an acoustical path from the other side of the diaphragm comprisesmeans deiining a chamber having an opening therein to the ambientatmosphere and a bore in said chamber defining means in communicationwith the chamber and the diaphragm, and a layer of acoustical resistancematerial disposed over the bore.

3. A microphone comprising the elements of claim 1 wherein the portionof the casing extending from one side of the diaphragm forming thetransmission line comprises an elongated tube having a wall defining achannel therein.

4. A microphone comprising the elements of claim 2 wherein the end ofthe tube opposite the transducer is provided with an opening foradmitting sound therein.

5. A microphone comprising the elements of claim 1 wherein theelectromechanical transducer comprises a cylindrical magnet mounted onfthe housing having an axial channel therein communicating and alignedwith the acoustical transmission line, a magnetic yoke mounted on themagnet on the end thereof adjacent the transmission line and forming acircular gap at the opposite end of the magnet, a diaphragm sealedacross the channel at the end thereof opposite the transmission line,and a voice coil disposed within the circular gap and mounted on thedlaphragrn,

6. A microphone comprising the elements of claim 5 in combination withan air impermeable plate on the yoke and disposed on the side of ,thediaphragm oppos1te the electromechanical transducer and spacedtherefrom, the perimeter of said plate being acoustically scaled to thehousing and said plate having a plurality of holes therethrough adjacentto the diaphragm, a layer of material forming an acoustical resistancedisposed over the holes.

7. A microphone responsive through a frequency range comprising anelectroacoustical transducer having a diaphragm, said transducergenerating an electrical signal responsive to the dilerence in pressuresexerted on opposite sides of the diaphragm, a casing enclosing saidtransducer and having a portion defining at least one acousticaltransmission line extending from one side of the diaphragm, said portionof the casing having a straight elongated hollow tube having a soundpermeable window disposed along an axis parallel to the axis of thetube, the sound permeable window admitting sound into the transmissionline at points located at a plurality of distances from the diaphragm,,thus forming a plurality of sound paths to the diaphragm of differentlengths, the longest of the sound paths being less than a halfwavelength longer than the shortest of said paths at the lowestfrequency of the response range, and a hollow cylindrical sleeve mountedcoaxially about the tube and spaced therefrom, said sleeve having anelongated slot parallel to the axis thereof confronting the tube, saidcasing including means delining an acoustical path extending fro-m theother side of the diaphragm and having an opening on said other side ofthe diaphragm, said path having an acoustical impedance disposed thereinand a `hand pass extending from the lowest frequency of `the range ofthe microphone 4to the frequency at which the longest of the paths tofthe transmission line is a half wavelength, the length of said path andthe magnitude of said impedance providing a directional response in theportion of the frequency range of the microphone covered by the bandpass of said acoustical path to said other side of the diaphragm.

8. A microphone having a frequency response band comprising, incombination, an air impermeable hollow housing having a cylindricalopening and a coaxial circular recess in the exterior surface of thehousing about the opening, a hollow cylindrical tube mounted on thehousing within the opening and extending from the housing, said tubehaving a plurality of spaced holes therethrough on a line parallel tothe axis of elongation thereof, the holes extending along the line adistance less than a half wavelength at the lowest frequency in theresponse band of the microphone, a hollow cylindrical sleeve disposedcoaxially about the tube and spaced therefrom, said sleeve being mountedin the recess of the housing and having a plurality of slots along twolines on opposite sides of the sleeve parallel lto .the axis ofelongation, said lines of slots being equidistant from the line of holesin the tube, a plug sealed within the end of the tube opposite thehousing having a bore extending therethrough, a sound permeable capmounted on the end of the sleeve opposite the housing confronting theplug, an electroacoustical transducer mounted within the housing havinga diaphragm sealed to the housing and confronting the tube on one side,and means defining an acoustical path coupled to the other side of thediaphragm, said path having -an opening on the side of the diaphragmopposite the tube and an acoustical impedance therein, `said meansdening an acoustical path to the other side of the diaphragm having aband pass extending from the lowest frequency of the range of themicrophone to the frequency at which the distance between the nearestand farthest holes of the tube from the housing is a half wave length.

9. A microphone having a frequency response band comprising, incombination, an air impermeable hollow housing having a cylindricalopening in the exterior surface thereof, a hollow cylindrical tubemounted on the housing within the opening and extending from thehousing, said tube having a plurality of spaced holes therethrough, thehole farthest from the housing being spaced from the hole closest to thehousing by a distance less than a half wave length at the lowestfrequency in the response band of the microphone, an electroacousticaltransducer mounted within the housing having a diaphragm sealed to thehousing and confronting the tube on one side thereof, means defining arst chamber confronting the other side of the diaphragm, including abore, and means dening a second chamber confronting the bore of thefirst chamber, said second chamber having an opening to the ambientatmosphere, and a layer of acoustical resistance material disposed overthe bore, said first and second chambers and acoustical resistancematerial defining an acoustical path to the other side of the diaphragmhaving a band pass extending from the lowest frequency of the range ofthe microphone to the frequency at which the distance between thenearest and farthest `holes of the tube from the housing is a half wavelengh.

10. A microphone having a frequency response band comprising, incombination, a hollow housing constructed of air impermeable materialhaving a cylindrical opening and a coaxial circular recess in theexterior surface of the housing about the opening, a hollow cylindricaltube mounted on the housing within the opening and extending from thehousing, said tube having a plurality of spaced holes therethrough on aline parallel to the axis of elongation thereof, the holes extendingalong the line a distance less than a half wavelength at the lowestfrequency in the response band of the microphone, a hollow cylindricalsleeve disposed coaxially about the tube and spaced therefrom, saidsleeve being mounted in the recess of the housing and having a pluralityof slots along two lines on opposite sides of the sleeve parallel to theaxis of elongation, said lines of slots being equidistant from the lineof holes in the tube, a plug sealed within the end of the tube oppositethe housing having a bore extending therethrough, a sound impermeablecap mounted on the end of the sleeve opposite the housing confrontingthe plug,

an electroacoustical transducer mounted within the housing including adiaphragm acoustically sealed about its perimeter to the housing anddisposed normal to the tube, said electroacoustical transducer alsoincluding a voice coil `mounted on the diaphragm coaxially with andconfronting the tube, a hollow cylindrical magnet mounted within thehousing coaxial with the tube and having one `end disposed adjacent tothe voice coil, and a ferromagnetic yoke having a plate provided with acircular aperture disposed about the voice coil and a cup-shaped portionextending from the perimeter of the plate to the end of the magnetopposite the voice coil, said cup-shaped portion having an aperturealigned with the tube, said housing having a plate disposed therein onthe side of the diaphragm opposite the tube dividing the housing intotwo chambers, said plate having a bore therein, a layer of resistancematerial disposed over the bore, and said housing having an opening fromthe exterior into the chamber thereof remote from the diaphragm, theopening, chamber remote from the diaphragm, bore, resistance material,and chamber adjacent to the diaphragm forming a low pass acousticaltransmission line for sound waves having a wavelength greater than twicethe length of the tube.

References Cited in the file of this patent UNITED STATES PATENTS2,299,342 Olson Nov. 30, 1939 2,529,228 Schwalm Nov. 7, 1950 2,789,651Daniels Apr. 23, 1957 2,848,561 Gorike Aug. 19, 1958 2,856,022 Kurtze etal Oct. 14, 1958 2,865,464 Gorike Dec. 23, 1958 2,921,993 Beaverson Ian.19, 1960 2,939,922 Gorike June 7, 1960 FOREIGN PATENTS 774,003 GreatBritain May l, 1957 796,032 Great Britain June 4, 1958 1,014,595 GermanyAug. 29, 1957 OTHER REFERENCES A New Cardioid Microphone, Friedman,Tele-Tech & Electronic Industries, October 1955, pp. -72, 129-Acoustical Engineering, Olson, 1957, Fig. 8.69, page 324.

1. A MICROPHONE RESPONSIVE THROUGH A FREQUENCY RANGE COMPRISING ANELECTROACOUSTICAL TRANSDUCER HAVING A DIAPHRAGM, SAID TRANSDUCERGENERATING AN ELECTRICAL SIGNAL RESPONSIVE TO THE DIFFERENCE INPRESSURES EXERTED ON OPPOSITE SIDES OF THE DIAPHRAGM, A CASING ENCLOSINGSAID TRANSDUCER AND HAVING A PORTION DEFINING AT LEAST ONE ACOUSTICALTRANSMISSION LINE EXTENDING FROM ONE SIDE OF THE DIAPHRAGM, SAID CASINGHAVING MEANS LOCATED AT A PLURALITY OF DISTANCES FROM THE DIAPHRAGM TOADMIT SOUND INTO THE TRANSMISSION LINE, THUS FORMING A PLURALITY OFSOUND PATHS TO THE DIAPHRAGM OF DIFFERENT LENGTHS, THE LONGEST OF SAIDPATHS BEING LESS THAN A HALF WAVELENGTH LONGER THAN THE SHORTEST OF SAIDPATHS AT THE LOWEST FREQUENCY OF THE RESPONSE RANGE, SAID CASINGINCLUDING MEANS DEFINING AN ACOUSTICAL PATH EXTENDING FROM THE OTHERSIDE OF THE DIAPHRAGM HAVING A CAVITY AND AN OPENING ON SAID OTHER SIDEOF THE DIAPHRAGM SPACED FROM THE DIAPHRAGM BY A DISTANCE LESS THANONE-HALF THE LENGTH OF THE LONGEST PATH OF THE TRANSMISSION LINE, SAIDPATH HAVING AN ACOUSTICAL IMPEDANCE DISPOSED THEREIN AND A BAND PASSEXTENDING FROM THE LOWEST FREQUENCY OF THE RANGE OF THE MICROPHONE TOTHE FREQUENCY AT WHICH THE LONGEST OF THE PATHS OF THE TRANSMISSION LINEIS A HALF WAVELENGTH, THE VOLUME OF SAID CAVITY AND THE MAGNITUDE OFSAID IMPEDANCE COMPRISING MEANS FOR PRODUCING AN ACOUSTICAL PHASESHIFTAPPROXIMATELY EQUAL TO THE SHIFT IN PHASE OF A SOUND WAVE IN SAIDPASS BAND TRAVELING BETWEEN THE DIAPHRAGM AND OPENING WHEREBY THEMICROPHONE ACHIEVES A DIRECTIONAL RESPONSE IN THE PORTION OF THEFREQUENCY RANGE OF THE MICROPHONE COVERED BY THE BAND PASS OF SAIDACOUSTICAL PATH TO SAID OTHER SIDE OF THE DIAPHRAGM.